As described in non patent literature 1, a scanning electron microscope (hereinafter abbreviated as SEM) is specified to a critical dimension or length measurement SEM which is dedicated to the semiconductor measurement while enjoying widespread use as a pattern dimension managing tool in the semiconductor process. The principle of the length measurement SEM is illustrated in FIG. 2. An electron beam 910 emitted from an electron gun 901 is converged by means of a condenser lens 902 and while being focused on the surface of a sample 900 by means of an objective lens 904, the electron beam is scanned two-dimensionally on the sample 900 with the help of a deflector 903 controlled by a controller 906. By capturing secondary electrons 920 given off from the sample 900 under irradiation of the electron beam 910 with the help of a detector 905 and causing them to undergo signal processing, an electron beam image as displayed on a CRT 907 can be obtained. Since the secondary electrons are generated more abundantly at a pattern edge portion, the image of electron beam results in a picture having a bright portion corresponding to the pattern edge as displayed on the CRT 907. In the length measurement SEM, the dimension can be determined as (1×p) by multiplying an inter-edge distance 1 (pixels) by a pixel size p (nm/pixel) on the electron beam image.
An example of length measurement process in the length measurement SEM is described in patent literature 1. In the example disclosed by the patent literature 1, from a local area inside an image resulting from image-picking up a measurement objective wiring conductor, a projective waveform is prepared which is obtained by adding and averaging signal waveforms on the wiring conductor in the longitudinal direction of the wiring conductor, and the dimension of wiring conductor is calculated as a distance between the bilateral wiring conductor edges which is detected in the projective waveform.
Out of a variety of methods proposed as methods of detecting edge positions for the sake of the automatic calculation of inter-edge distance 1, a threshold value method widely used in general will be described hereunder.
The threshold value method is disclosed in, for example, patent literature 2. As shown in FIG. 3, peak portions of large signal amounts corresponding to bilateral pattern edges will be called a left white band (left WB) and a right white band (right WB), respectively. In the threshold value method, a Max value and a Min value are determined at each of the right and left WB's, a threshold level which divides internally the Max and Min values at a predetermined ratio th (%) is calculated and a crossing point of the threshold value and a signal waveform is defined as an edge position.
With a semiconductor device pattern rendered corpuscular, the demand for measurement accuracy is becoming stringent year by year. In the case of the length measurement SEM, a plurality of length measurement SEM's are thrown into one semiconductor fabrication line and they are often used in combination and accordingly, not only the measurement reproducibility of a single length measurement SEM but also the measurement reproducibility between devices, that is, differences among measurement values of the plural length measurement SEM's (hereinafter referred to as machine differences) come into question. The machine difference is demanded to be less than 0.18 nm for DRAM half pitch 45 nm generation, less than 0.13 nm for DRAM half pitch 32 nm generation and less than 0.09 nm for DRAM half pitch 22 nm generation, as exemplarily described in the non patent literature 2.
The non patent literature 2 also discloses a method for measuring the machine difference. As described above, differences in measured values among the plural SEM's are defined as the machine differences but in the length measurement SEM, a sample is caused to change by undergoing contamination due to electron beam irradiation (a phenomenon in which an amorphous carbon film is deposited on a portion of the sample irradiated with the electron beam), sample electrification or shrink (shrinkage of a resist pattern due to electron beam irradiation), with the result that even when the same measurement objective is measured with the plural devices and differences among the measured values are simply determined, the machine difference cannot be said as being measured correctly. This is because the difference between measured values is inevitably affected by the change in sample. The non patent literature 2 shows a concrete method for dealing with the problem as above. Two typical expedients, namely, an ABBA method and a grating method will be described hereinafter.
(1) ABBA Method
Defined are a machine difference δ (nm) between devices B and A, a measurement value change amount c1 (nm) due to contamination, sample electrification and shrink caused by measurement in a device A and a measurement value change amount c2 (nm) caused by measurement in a device B. The machine difference is determined through steps 1 to 3 as described in the following.
Step 1: In respect of a plurality of spots (usually, several of tens of spots) on the sample, measurement is conducted with the use of device A, followed by measurement with the use of device B (AB sequence) and an average of values measured by the device A is subtracted from an average of values measured by the device B to obtain a value δ1 (nm). The relation is held among δ, δ1 and c1 as expressed by a mathematical expression 1.δ1=δ+c1  (MATH. 1)Step 2: In respect of a plurality of spots (usually, several of tens of spots) on the sample, measurement is conducted with the use of device B, followed by measurement with the use of device A (BA sequence) and an average of values measured by the device B is subtracted from an average of values measured by the device A to obtain a value δ2 (nm). The relation is held among δ,δ2 and c2 as expressed by a mathematical expression 2.δ2=−δ+c2  (MATH. 2)Step 3: Assuming that c1=c2 stands (changes in sample caused by each of the devices A and B are substantially identical), from mathematical expressions 1 and 2, δ can be determined byδ=(δ1−δ2)/2  (MATH. 3)
According to the ABBA method, a correct machine difference can be obtained when the assumption of c1=c2 stands (indicating that changes in sample caused by the respective devices A and B are deemed to be substantially identical) and the number of measurement spots is large enough by taking measurement errors in individual measurements into consideration.
(2) Grating Method
In the grating method, for the sake of avoiding the influence the changes in sample due to the electron beam irradiation, the same spot is not measured plural times but locations of measurement spots during measurement of machine differences among four devices (A, B, C, D) are displaced from one another so that each device may measure many fresh spots (spots not image-picked up even once) of each of the devices and a difference between averages may be defined as a machine difference.
In this case, with a view to permitting a distribution of completed patterns on plane to hardly have an influence upon measurement spots, the individual devices have measurement spots which are so arranged as not to localize.